Laser annealing (also called laser spike annealing, laser thermal annealing, laser thermal processing, etc.) is used in semiconductor manufacturing for a variety of applications, including for activating dopants in select regions of devices (structures) formed in a semiconductor wafer when forming active microcircuits such as transistors and related types of semiconductor features.
One type of laser annealing involves the formation of a line-shaped intensity profile that is scanned over the wafer by moving the line image, moving the wafer, or a combination of these two movements. The line image is scanned in a “scan direction,” which is perpendicular to its long axis. Some spatial variation in the intensity in the line image along the scan direction (i.e., the short axis of the line image) can be tolerated since the non-uniformities are averaged out as the line image moves over the wafer. On the other hand, the spatial variation of the intensity profile in the “cross-scan” direction of the line image needs to be tightly controlled to achieve consistent annealing results over the scan path of the line image.
One approach to defining the length of a line image involves passing light through an aperture defined by opposing knife edges. The light passing through the aperture is then imaged by a relay optical system to the wafer. In this type of line-imaging optical system, it has been observed that the intensity profile of the line image in the long direction undergoes time-varying changes. In many cases the changes manifest as a “tilt” (i.e., change in slope) in the intensity profile, and over time this tilt may shift, with the profile slope varying in time and even changing sign (e.g., the slope may change from positive to negative). This type of beam profile instability is also referred to as “beam wobble.” This phenomenon can reduce the uniformity of the laser annealing process.